1. Field of the Invention
The present invention relates to an X-ray CT (computerized tomography) scanning apparatus and more particularly, to an X-ray CT apparatus capable of reducing undesired artifact in a reconstructed image caused by the action of helical scanning.
2. Prior Art
(1)Single Slice CT
X-ray CT scanning apparatuses of the single slice type are commonly used which includes an X-ray source 101 for irradiating a fan-shaped beam of X-ray (a fan beam) and a detector 103 composed of an M number, e.g. 1000, of detector elements aligned in an arcuate or linear row, as shown in FIG. 1A. The X-ray source 101 and the detector 103 are rotated in a combination around an object to be scanned, as shown in FIG. 1A, for sampling intensity information (referred to as projection data hereinafter) of X-ray beams which have passed through the object. For example, projection data of 1000 times are sampled from one cycle of the irradiation and used for reconstruction of a visual image which will be explained later. It should be assumed that one time of the irradiation is termed one view, data of each detector element in one view is called one beam, and all the beams in one view are combined to a real data (from the detector elements).
(2)Two Different Scanning Methods
Two different scanning methods on the X-ray CT apparatus will now be explained.
One of the two scanning methods is of such a conventional type as shown in FIG. 2A where a cross section A is scanned by the source 101 which travels one full circle. If another cross section B is required to be scanned in addition to the cross section A, either a couch on which the object to be scanned is placed or a combination of the X-ray source 101 and the detector 103 is shifted to the cross section B after the completion of scanning the cross section A. The data is then collected by the combination traveling around the object. This will require a considerable length of time for scanning a series of cross sections of the object which is extended lengthwisely (along the Z axis).
The other scanning method is of such a helical scan type as shown in FIG. 2B where the X-ray source 101 and the detector 103 are rotated in circles while the couch travels lengthwisely of the object to be scanned in synchronization with the rotating movement. This allows the X-ray source 101 to scan the object as travel along a helical path. The helical scanning method is faster in the scanning speed and broader in the scanning area.
It is also assumed that the coordinate system involved is as shown at the left in FIG. 7C. The X-Y plane is a plane to be scanned by the conventional manner where the cross sections A and B fall. The Z axis is equivalent to a lengthwise direction of the object to be scanned or a slice direction in the single slice type CT scanning.
(3)Reconstruction of Conventional Scanned Image
Reconstruction of a scanned image sampled by the X-ray CT apparatus is explained below in brief. The procedure related to the conventional scanning manner consists of three steps. It is assumed that the object to be scanned is translated to a signal across the center of rotation denoted by the solid arrow shown at the upper left in FIG. 3.
[1]Sampling and compensation of data
The data is sampled by the conventional scanning method. Although the angle of rotation is shown through 90 degrees in the drawings, 360 or 180+any other fan shaping angle may be employed. Resultant patterns of projection data are as shown at the upper right in FIG. 3 and then compensated with relevant factors including the sensitivity of the detector 103, the intensity of X-ray beams, and other parameters, thus producing a raw data.
[2]Convolution with reconstructing function
The raw data sampled from all the angles are subjected to convolution of a reconstructing function. Patterns of the resultant convoluted data are shown at the lower right in FIG. 3 as have a decay at the original signal.
[3]Back projection
The convoluted data is then added to the pixels which are arrayed along the path of an X-ray beam. A result of the back projection is shown at the lower left in FIG. 3. By repeating the back projection for the convoluted data at all the angles, the original signal is reconstructed.
(4)Reconstruction of Helical Scanned Image
FIGS. 4A and 4B illustrate side-viewed patterns generated by the two, conventional and helical, scanning methods of FIGS. 2A and 2B respectively. The horizontal axis represents the slice direction (along the Z-axis) and the vertical axis is a phase (angle) of the rotation. The sampling locations are denoted by the arrows. Those drawings are referred to as "scan diagrams".
In the conventional scanning method of FIG. 4A, data at the target slicing location is sampled through 360 degrees of the phase by the prescribed step of [1] allowing the reconstruction of an image in the steps [1] to [3].
The helical scanning method of FIG. 4B however permits a helical scanning action which samples one view at the target slicing location. It is hence needed for interpolation along the Z-axis of the raw data instead of the step [1] before the steps [2] and [3] are executed. Typical methods of the interpolation for the single-slice type CT scanning are as follows:
(a) 360-degree interpolation
The 360-degree interpolation is shown in FIG. 5A where the real data of two views which are in phase with each other and designated on both sides of the target slicing location are used for linear interpolation with an inverse of the ratio of distance between the slicing location and the sampling location.
For example, if the target slicing location is expressed by Z=Z0 (at the z coordinate point of the slicing plane), its scanned data is one view at zero of the phase. For sampling data at a phase point .theta., the two real data 1 and 2 on the upper and lower sides of the slicing location respectively are selected and used for linear interpolation with an inverse of the ratio of distance between their sampling location and the slicing location Z(in the z coordinate). By repeating this process, the interpolated projection data throughout the phase is obtained.
(b)Counter beam interpolation
This employs an opposite beam of imaginary data, so called "180.degree. LI". If the X-ray source is located at the black point as shown in FIG. C, scanning beams incident on the detector elements are denoted by the arrows of the real line. The irradiating path of the beam 1 at the left side is identical to that of a beam from the X-ray source located at the white point. The beam irradiated from the white point and denoted by the dotted arrow is hence called an opposite beam. Similarly, the opposite beams of the beams 2 and 3 are irradiated from the gray point and the dark gray point respectively as also denoted by the dotted arrows. All the scanning beams irradiated from the black point are opposite to their respective opposite beams. This allows imaginary data of the opposite beams to be produced from data of the X-ray source from the white to dark gray points (as thus referred to as counter data). The opposite beam interpolation performs linear interpolation between the real data and its opposite data.
In the helical scanning, the sampling location for opposite data is varied depending on a beam (or a channel) as shown in FIG. 5D. For ease of the description, the sampling location is represented by the center channel denoted by the dotted line as shown in FIG. 5B. The interpolation of data sampled by the helical scanning may be implemented with the use of non-linear functions of which methods are also known.
(5)Slice Profile and Image Quality
Two of primary factors representing the performance of the system are slice profile and image quality. The slice profile is a impulse response along the Z-axis or (slice) direction. Its example is shown in FIG. 6 where an ideal slice has a square-like profile and its effective thickness (a width on a half of a value I.sub.O) W.sub.as is small. More specifically, the profile 1 is identical in the effective slice thickness W.sub.as to the profile 2 but is more favorable in the shape than the same. The profile 2 is smaller in the effective slice thickness W.sub.as and thus more favorable than the profile 3.
As shown in FIG. 5A, the distance between two sampling locations of data to be used for the interpolation is termed an interpolation interval. The interpolation interval is equal to the helical pitch in the 360-degree interpolation and 1/2 the helical pitch in the opposite beam interpolation. The smaller the interpolation interval, the thinner the effective slice thickness W.sub.as is in the helical scanning action. Accordingly, the opposite beam interpolation is preferable.
(6)Multi-slice type CT scanning
For scanning a wider area at a higher resolution, a multi-slice type of CT scanning is provided having multiple rows, e.g. two, four, or eight rows, of detector elements as shown in FIGS. 7A, 7B, and 7C. FIG. 8A illustrates a fan beam viewed from the Z direction in which the center circle is an effective field of view (FOV). FIG. 8B explains four-row multi-slice CT scanning viewed from a direction vertical to the Z axis. The fundamental slice thickness T is defined by the width of a Z-axial beam at the center of rotation (spaced by a distance FCD from the X-ray source) which is irradiated from the X-ray source to a row of the detector elements.
(7)Helical Scanning of Multi-Slice Type CT Mode
The helical scanning of the multi-slice type CT mode is disclosed in Japanese Patent Laid-open Publication 4-224736 (1992). It is said that the helical pitch P of the multi-slice type CT scanning is substantially similar to the pitch of the single-slice type CT scanning and equal to a product of the number N of rows of the detector elements and the fundamental slice thickness T or a total slice thickness Ta at the center of rotation, as is expressed by: EQU P=N.times.T (1)
In this description, the helical pitch will be expressed hereinafter by a value calculated by dividing P by T. From the equation (1), the helical pitch is 4.
As depicted in the above publication, one of the interpolation methods for the helical scanning with the pitch of N in the N-row multi-slice type CT mode a modification of the 360-degree interpolation of the single-slice CT mode.
FIG. 9 is a scan diagram in which the four-row multi-slice type CT scanning employs such a modification of the interpolation. More particularly, similar to the 360-degree interpolation shown in FIG. 5A, the interpolation is carried out between two real data sampled at both sides of the target slicing location. This modification is now called an adjacent interpolation. Since the interpolation interval is equal to the fundamental slice thickness as alike in the 360-degree interpolation, the effective slice thickness W.sub.as is also similar to that of the 360-degree interpolation.
In the interpolation for the above (7) scanning, the effective slice thickness W.sub.as is not small. Hence, the opposite beams may be used for decreasing the effective slice thickness W.sub.as.
FIG. 10A shows the helical scanning with an opposite beam at a pitch of 4, in which the opposite beam is denoted by the shaded area: a first rotation by the diagonal hatching and a second rotation by the vertical hatching. As apparent, most of the shaded area overlaps the sampling location of real data. Opposite data from scanning with the opposite beam is specified by the black rectangular area corresponding to the real data at the blank point in the single-slice type CT scanning. However, a majority of the opposite data is located on the same side of the target slicing location as of the real data. This requires extrapolation and if the opposite beam is desired without interruption, it may depart from the target slicing location depending on the channel or path. As the result, the extrapolation will increase generation of error and the departure from the target slicing location will cause the effective slice thickness W.sub.as to increase.
This drawback may be compensated by the opposite beams located opposite to the real data being sampled for opposite data as shown in FIG. 10B. Although no extrapolation is involved hence causing minimal error, there is generated an interruption between two adjacent beams in weighted interpolation. Consequently, the quality of a reconstructed image will be declined due to the interruption.